Introduction

The prevalence of obesity has increased dramatically during the past two decades, reaching epidemic proportions (1,2). In Denmark, the prevalence of obesity among young men has increased almost 50-fold since 1960 (3,4). To prevent and treat overweight and obesity, public-health strategies have aimed to promote both an increase in people's daily physical activity and dietary changes that reduce the intake of fatty foods and sugared beverages. Most of our experience regarding the efficacy of weight-management strategies comes from clinical trials, and it is well known that most obese individuals can lose weight, but regain the weight within 5 years. However, many slimming efforts are exerted among consumers outside of the clinical setting. A large variety of diets, activity programs, supplements, herbal compounds, and more exotic, unsubstantiated treatments are available. Our knowledge about the use of these weight-loss aids is limited. However, recent research has reported a prevalent use of nonprescription weight-loss products by American women (5).

The aim of the present study was to evaluate the slimming behavior of the adult population in Denmark from two independent telephone interviews conducted in 1992 and in 1998. Initially, we wanted to examine the overall slimming prevalence in Denmark during the 1990s and to analyze its dependence on the subjects' gender, age, and weight status. Furthermore, we wanted to make a comparison of the two surveys to see whether the choice of slimming methods haschanged from 1992 to 1998, as well as to see whether the subjects' gender, age, and body mass index (BMI) influenced the slimming method choice. Finally, it was of interest to examine whether weight loss was achieved and whether this weight loss was maintained.

Research Methods and Procedures

Design

This retrospective study was based on two independent surveys that were carried out in Denmark in January 1992 and in September 1998 by Sonar (Institute of Market and Opinion Analyses, Hørsholm, Denmark) on behalf of the daily newspaper Jyllandsposten. The subjects were selected on a random basis according to their social security number. The surveys were designed to ensure an equal gender distribution and stratification on age groups and geographic regions in Denmark. The subjects ( 17 years) were interviewed by telephone; 8.0% and 14.0%, respectively, refused to participate, giving 1239 respondents in 1992 and 1207 respondents in 1998. Descriptive characteristics of the participating subjects are presented in Table 1.

Table 1 - Descriptive characteristics of Danish adults (n = 2446), 17 years of age and older, participating in the two surveys conducted in 1992 (n = 1239) and in 1998 (n = 1207).

Full table

Questionnaire

The same questionnaire (Appendix) was used in both surveys and provided information about the gender and age of subjects, their experiences regarding weight-loss attempts, and when the last weight-loss attempt had taken place. Furthermore, the subjects were asked to answer questions about the most recent method(s) used to achieve weight loss ("changing the habitual diet," "slimming with supervision of a physician," "over-the-counter diet pills or meal replacements," "increased exercise," and "other methods"). Finally, the questionnaire gave information about weight-loss achievement after the last terminated slimming treatment, maintenance of the weight loss in question, and the present weight and height of the subjects. With regard to the question concerning slimming-method choice, the subjects could choose more than one answer (i.e., slimming method), and this was considered in the statistical analyses.

Data Processing

All subjects (n = 2446) were included in the statistical analysis of slimming prevalence (question 1 from the questionnaire). Of the subjects in the 1992 and 1998 surveys, 682 (55%) and 644 (53%), respectively, had never tried slimming and were therefore excluded from further analyses, which focused on the choice of slimming method(s), weight-loss achievement, and weight-loss maintenance (questions 3, 4, and 5 from the questionnaire, respectively). The first three response options in question 2 regarding the time of the last slimming treatment ("slimming at the moment," "within the last 14 days," and "within the last month") were excluded when analyzing weight-loss achievement (question 4) and weight-loss maintenance (question 5). With regard to the question about weight-loss achievement (question 4), the two response options, "the desired weight loss" and "less weight loss than expected," were categorized as "obtained weight loss," whereas a negative outcome of the slimming treatment was categorized as "none or almost no weight loss" by merging the two response options: "none or almost no weight loss" and "I gained weight instead of losing weight." The final response option in question 4, "the slimming treatment was not completed," was excluded from the statistical analysis of weight-loss achievement. In the analysis of weight-loss maintenance (question 5), only respondents who had obtained weight loss were taken into account. The final two response options to question 5 ("gained weight gradually again" and "gained weight quickly again") were merged in the statistical analyses, and the responses to question 5 were therefore categorized as "weight loss maintained" and "weight loss not maintained".

In all statistical analyses, the age of the respondents was classified in the following three groups: subjects <30 years; subjects 30 to 50 years; and subjects >50 years.

The WHO classification of BMI was used: underweight, <18.5 kg/m²; normal weight, >18.5 kg/m² to <25.0 kg/m²; overweight, 25.0 to <30.0 kg/m²; and obesity, 30 kg/m². Due to the low number of underweight individuals having attempted weight loss (Table 2), the underweight and normal-weight individuals were merged in the statistical analyses of weight-loss achievement and weight-loss maintenance.

Table 2 - Descriptive characteristics of Danish adults (n = 2446) 17 years of age and older participating in the two surveys conducted in 1992 (n = 1239) and in 1998 (n = 1207).

Full table

Statistical Analyses

Logistic regression analysis (SAS 8.1; PROC GENMOD; SAS Institute, Cary, NC) was used for the initial analysis to test whether the proportion of subjects attempting or ever having attempted weight loss differed in the two surveys and/or varied significantly with the variables of age, gender, and BMI. In the analysis, all first-order interactions of the variables were included.

We tested whether the use of the five slimming methods differed between the two surveys or varied with age, gender, and BMI by a logistic regression analysis, in which all first-order interactions among the variables were included. Logistic regression was used in the analysis of how the immediate effect of the slimming treatment (categorized as "obtained weight loss" and "none or almost no weight loss") was related to the methods of slimming. Likewise, a logistic regression analysis was conducted for the analysis of any sustained effect of the slimming treatment as reported by the subjects (categorized as whether or not the weight loss was maintained). In both analyses, all first-order interactions of the various slimming methods were included. Furthermore, the variables gender, age, BMI, and year of interview were included to adjust for a possible confounding effect.

In all statistical analyses, the variables age and BMI were included as grouped variables for consistency and to obtain a better model fit.

Results

Slimming Prevalence

Table 2 presents the distribution of slimming subjects (n = 1120) with regard to gender and BMI for the surveys in 1992 and 1998. The results of the statistical analyses on slimming prevalence (question 1) are given in Table 3. Of the total number of women (n = 1188) and men (n = 1258), more women (61%) than men (32%) had attempted weight loss at least once in their lifetime [odds ratio (OR) = 5.18, p < 0.0001; Table 3]. Weight-loss attempts were influenced by age, as indicated by slimming prevalence being more prevalent in subjects <30 years (49%) compared with subjects >50 years (38%) (OR = 2.77, p < 0.0001) and in subjects 30 to 50 years (53%) compared with subjects >50 years (OR = 2.66, p < 0.0001). No significant difference between the age groups of individuals <30 years and 30 to 50 years was detected. Slimming prevalence increased with increasing BMI (p < 0.0001; Table 3), as indicated by 25% of the underweight, 38% of the normal-weight, 54% of the overweight, and 77% of the obese individuals having attempted weight loss at least once in their lives. All pairwise comparisons within the BMI groups were significant (p < 0.0001; Table 3).

Table 3 - Prevalence of slimming (question 1 from the questionnaire) as assessed by the subjects' answers from the two surveys conducted in 1992 (n = 1239) and in 1998 (n = 1207).

Full table

Slimming Methods

Figure 1 presents the choice of slimming method(s) (question 3) distributed among the year of interview, gender, BMI, and age groups, respectively, and statistical details are given in Table 4.

Figure 1.

Characteristics of slimming-method use as assessed by the subjects questioned (question 3 from the questionnaire). The total number of subjects slimming was 1120, distributed among the following categories: 1992, n = 557; 1998, n = 563; men: n = 401; women, n = 719; <30 years, n = 212; 30 to 50 years, n = 511; >50 years: n = 397; underweight (UW; BMI < 18.5 kg/m²), n = 15; normal weight (NW; BMI, >18.5 to <25.0 kg/m²), n = 544; UW + NW (BMI <25.0 kg/m²), n = 559; overweight (OW; BMI 25.0 to <30.0 kg/m²), n = 431; obese (OB; BMI 30 kg/m²), n = 130. The distribution of slimming-method responses were as follows: change of the habitual diet, n = 837; physician supervision, n = 98; over-the-counter pills or meal replacements, n = 145; increased exercise, n = 372; and other methods, n = 62. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; †, over-the-counter diet pills or meal replacements: interaction between year and age group (p < 0.05); n.s., not significant (p > 0.05); y, years. Statistical details are given in Table 4 .

Full figure and legend (106K)

Table 4 - Use of slimming methods in 1992 and in 1998 (question 3 from the questionnaire): comparison and variation with year of interview, age group, gender, and BMI.

Full table

The proportion of slimming subjects who reported having changed their habitual diet during their last slimming treatment was higher in 1998 (77%) than in 1992 (72%) (OR = 1.36, p < 0.05). The choice of dietary change as a slimming method was also significantly dependent on gender, as indicated by 79% of men and 73% of women having changed their habitual diet during their last slimming treatment (OR = 1.62, p < 0.01). The choice of dietary change decreased with increasing BMI, as indicated by 78% of the underweight and normal-weight, 73% of the overweight, and 66% of the obese individuals having reported the use of this method (p < 0.001).

Individuals choosing to have their physician supervise their last slimming treatment increased from 1992 (6%) to 1998 (12%) (OR = 1.93, p < 0.01). Furthermore, this slimming method was dependent on gender and increased with increasing BMI (Figure 1 and Table 4). The use of physician supervision as a slimming method was more prevalent in women (10%) than in men (7%) (p < 0.001, OR = 2.54). With regard to BMI, 3% of slimming underweight and normal-weight, 10% of slimming overweight, and 28% of slimming obese individuals had their physician supervise their last slimming treatment (p < 0.0001).

The use of over-the-counter pills or meal replacements as a slimming method decreased from 1992 (15%) to 1998 (11%) (Figure 1), a decrease that was found particularly in subjects 30 to 50 years (OR = 2.69, p < 0.001) (Table 4), as indicated by 19% and 9% of the subjects in this age group having reported the use of this mode of slimming in 1992 and 1998, respectively. The use of over-the-counter diet pills or meal replacements was more prevalent in women (15%) than in men (10%) (p < 0.01, OR = 1.94) (Table 4), and it was more common in obese (16%) (p < 0.01, OR = 2.32) and overweight (15%) (p < 0.001, OR = 2.18) individuals than in the underweight and normal-weight (10%) individuals (Table 4).

The use of increased exercise as a slimming method did not change between the two surveys, as indicated by 33% and 34% of subjects having reported the use of this slimming method in 1992 and 1998, respectively. The use of increased exercise was dependent on gender, as indicated by more men (37%) than women (31%) reporting the use of this method in their last slimming treatment (p < 0.01, OR = 1.47). A difference between age groups was observed (p < 0.0001), which was due to a more prevalent use of increased exercise in the younger age groups, as indicated by 50% of the subjects <30 years, 34% of the individuals 30 to 50 years, and 24% of the subjects >50 years having chosen increased exercise in their last slimming treatment (Figure 1; Table 4). More underweight and normal-weight (38%) (p < 0.001, OR = 2.52) and overweight individuals (32%) (p < 0.01, OR = 2.01) reported the use of increased exercise in their last slimming treatment than did obese individuals (19%) (Table 4).

Weight-Loss Achievement

The question regarding weight-loss achievement (question 4) was answered by 798 subjects, of whom 702 reported weight loss, which corresponded to 63% of the slimming subjects. Figure 2 shows the proportion of subjects who obtained weight loss distributed among slimming methods, the year of interview, gender, BMI, and age groups, respectively. Statistical details are presented in Table 5. The proportion of slimming subjects reporting weight loss was 61% in 1992 and 64% in 1998, respectively (not significant; Figure 2).

Figure 2.

Characteristics of weight-loss achievement (left bars) and weight-loss maintenance (right bars) (questions 4 and 5 from the questionnaire, respectively), as assessed by the respondents. The total number of subjects slimming (n = 1120) are distributed among the following categories: 1992, n = 557, 1998, n = 563; men: n = 401; women: n = 719; <30 years, n = 212; 30 to 50 years, n = 511; >50 years, n = 397; underweight (UW; BMI <18.5 kg/m²), n = 15; normal weight (NW; 18.5 to <25.0 kg/m²), n = 544; UW + NW (BMI <25.0 kg/m²), n = 559; overweight (OW; 25.0 to <30.0 kg/m²), n = 431; obese (OB; BMI 30 kg/m²), n = 130. Distribution of slimming-method responses: change of habitual diet, n = 837; physician supervision, n = 98; over-the-counter pills or meal replacements, n = 145; increased exercise, n = 372; and other methods, n = 62. Weight-loss achievement: number of respondents taken into account, 798, of whom 702 reported weight loss (desired weight loss or less weight loss than expected), corresponding to 63% of slimming subjects (n = 1120). Weight-loss maintenance: number of respondents taken into account, 693, of whom 343 reported weight maintenance, corresponding to 31% of slimming subjects. In the analyses of weight-loss maintenance, only respondents who obtained weight loss were taking into account. *p < 0.05; **p < 0.001; ***p < 0.0001; †, over-the-counter diet pills or meal replacements were negatively associated with weight-loss achievement (p < 0.0001) and weight-loss maintenance (p < 0.0001); n.s., not significant (p > 0.05). Statistical details are given in Tables 5 and 6 .

Full figure and legend (75K)

Table 5 - Weight loss achievement after the last terminated slimming treatment (question 4 from the questionnaire) as assessed by the subjects questioned.

Full table

Weight-loss achievement was dependent on gender, as indicated by men (66%) reporting to be more successful than women (61%) (p < 0.05, OR = 1.66; Table 5). Furthermore, weight-loss achievement was dependent on the body-weight status of the subjects, as indicated by more underweight and normal-weight (68%) individuals than overweight individuals (61%) (p < 0.001, OR = 2.34) and obese individuals (46%) (p < 0.001, OR = 3.39), reporting to have lost weight during their last slimming treatment (Table 5). With regard to slimming methods, over-the-counter diet pills or meal replacements were associated with a significantly negative outcome (p < 0.0001, OR = 2.82).

Weight-Loss Maintenance

The question regarding weight-loss maintenance (question 5) was answered by 693 subjects, of whom 343 reported weight-loss maintenance, which corresponded to 31% of the slimming subjects.

Figure 2 shows the proportion of subjects who maintained their weight loss distributed among slimming methods, year of interview, gender, BMI, and age groups. Statistical details are presented in Table 6. The proportion of slimming subjects reporting weight-loss maintenance was 32% in 1992 and 30% in 1998, respectively (not significant; Figure 2).

Table 6 - Weight loss maintenance after the last terminated slimming treatment (question 5 from the questionnaire) as assessed by the subjects questioned.

Full table

Gender influenced weight-loss maintenance, as indicated by 32% of the men and 30% of the women having succeeded in maintaining their weight loss (p < 0.05, OR = 1.60; Table 6). Furthermore, weight-loss maintenance was dependent on the body-weight status of the subjects, as indicated by more underweight and normal-weight individuals (45%) (p < 0.0001, OR = 12.72) and overweight individuals (20%) (p < 0.05, OR = 2.77) having reported maintaining their weight loss during their last slimming treatment than obese individuals (6%; Table 6). With regard to slimming methods, weight-loss maintenance seemed to be negatively associated with the use of over-the-counter diet pills or meal replacements (p < 0.001, OR = 2.99; Table 6).

Discussion

The Influence of the Year of Interview, BMI, Gender, and Age on Slimming Prevalence, Weight-Loss Achievement, and Weight-Loss Maintenance

The Year of Interview.

The proportion of subjects having attempted weight loss at least once in their lives did not change from 1992 to 1998. This is surprising because the overall BMI distribution of subjects participating in the two surveys (n = 2446) showed that the prevalence of overweight and obesity increased from 1992 (overweight, 30%; obesity, 6%) to 1998 (overweight, 35%; obesity 8%) (Table 1). With regard to weight-loss achievement and weight-loss maintenance, no differences between 1992 and 1998 were detected.

BMI.

Of the total number of subjects slimming (n = 1120), 25% of the underweight and 38% of the normal-weight subjects had been on a slimming treatment. We cannot exclude the possibility that some of them were previously overweight and had reduced their body weight to the current normal-weight or underweight status. However, because this group of subjects corresponds to 50% of the total number of slimming individuals in the study, we find it more likely that a large proportion of both normal-weight and underweight individuals are actually slimming regularly. This is also supported by the fact that as much as 19% of the underweight and normal-weight subjects who had attempted weight loss were actually slimming when answering the questionnaire used in this study, and 31% of these individuals had attempted weight loss within the previous 3 months. Several studies have shown that body dissatisfaction is a very common phenomenon among normal-weight and even underweight women (6). They describe themselves as fat (7), and express a desire to become thinner (8). Of the obese and overweight individuals, 24% and 46% had never attempted weight loss.

Gender.

Almost twice as many women than men had attempted weight loss, although obesity was more prevalent in men (8%) than in women (6%). Men reported being more successful in achieving and maintaining weight loss than women. This observation might be determined from a psychological perspective because men might evaluate the results of a slimming treatment in a more positive and optimistic way than women, a phenomenon that could be explained partly by the disseminated body dissatisfaction in women as described above (6,7,8).

Age.

Overall slimming attempts were more prevalent in subjects <50 years, although obesity occurred more often in individuals >50 years (8%) compared with subjects <50 years (5%). It is well established that the serious health complications of obesity—e.g. type 2 diabetes—are increased by age; therefore, it could be expected that the elderly might be more engaged in slimming behavior. However, our findings are inconsistent with this expectation, findings that are most likely due to weight-loss attempts driven by cosmetic reasons rather than health concern. With regard to weight-loss achievement and weight-loss maintenance, no differences between these age groups were found.

Slimming Methods

Among the slimming methods examined in this study, a change of the habitual diet was by far the most prevalent mode of slimming, and the proportion of slimming individuals using this method increased from 1992 to 1998. This rise might be attributed to the Danish public health advice, which has focused on a reduction in dietary fat based on evidence from intervention studies (9). The use of dietary change as a method of slimming was more frequent in men than in women, as well as in underweight and normal-weight individuals compared with obese individuals.

The present study indicates that slimming under the supervision of a physician doubled from 1992 to 1998. The term "supervision of a physician" included both slimming under the guidance of a physician and the use of weight-loss prescription drugs. The use a physician's supervision was more prevalent in women than in men and increased with increasing BMI, which is consistent with studies from other countries (10,11).

The use of over-the-counter-diet pills or meal replacements decreased from 1992 to 1998, a decrease that was found particularly among the younger age groups. In 1992, the use of over-the-counter diet pills or meal replacements as a mode of losing weight were quite frequent among individuals <50 years, whereas no differences between age groups were observed in 1998. The overall decrease suggests a positive trend because this study indicated that the use of over-the-counter diet pills or meal replacements was negatively associated with weight-loss achievement and weight-loss maintenance.

The use of increased exercise was quite prevalent in both surveys (33% and 34% of slimming subjects in 1992 and 1998, respectively). This finding, together with the increase in changing habitual diet and decrease in using over-the-counter diet pills or meal replacements as modes of losing weight, could be due to the efforts of Danish scientists, the National Food Administration, and the media in the 1990s to focus more on dietary habits and physical activity as strategies to achieve and maintain weight loss. At the same time, criticism has been raised against the use of unsubstantiated over-the-counter slimming remedies and The Danish Nutrition Council has warned against the use of very-low-energy diets with an energy content of <800 kcal/day. Despite the large use of dietary change and increased exercise as an aid to lose weight, neither weight loss nor weight-loss maintenance could be attributed to these modes of slimming.

Men reported to change their habitual diet and to use increased exercise more often than women when trying to achieve weight loss, whereas the use of over-the-counter-diet pills or meal replacements and physician supervision was more prevalent among women. These findings suggest that the body-weight management strategies mentioned above have become more widespread in men compared with women.

In this study, the subjects were initially interviewed by telephone. This could imply a higher non-response rate in overweight and obese individuals than in non-obese subjects as found in a previous analysis of the response to a health examination (12). However, due to the high response rates of the surveys in the present study, we consider it to be unlikely that this selection bias has influenced the outcome of the present study.

In this study, height and weight of the subjects were not measured but self-reported. A number of studies have reported that self-reported weight and height is associated with a misreporting error, as people very often under-report their true weight, whereas height is often over-reported when compared to measured height (13,14,15,16,17,18,19,20,21). It is likely that the misreporting of height and weight also have occurred in these surveys, but it is unlikely that the errors were different in 1992 and 1998. Therefore, the relative changes in BMI and in prevalence of weight categories are likely to be valid.

The present study is an observational survey, and, consequently, we cannot draw final conclusions about the methods of slimming and the observed effect on weight loss and maintenance. Likewise, we do not know the weight history of the subjects, which limits the assessment of the relation of body-weight status to weight loss and weight-loss maintenance.

In conclusion, slimming behavior is very common in both genders, all adult age groups, and in all BMI strata. The proportion of subjects having attempted weight loss did not change from 1992 to 1998, although the prevalence of overweight and obesity increased from 1992 to 1998. About half of all adult Danes have been on a slimming program, particularly women and individuals <50 years. This finding is inconsistent with the fact that overweight and obesity are more prevalent in men and in individuals >50 years. Among the slimming methods examined in this study, change of habitual diet and increased exercise are by far the most prevalent modes of slimming, and the proportion of slimming individuals using dietary change increased from 1992 to 1998. The use of over-the-counter diet pills decreased from 1992 to 1998. Because this slimming method is associated with a negative outcome, this reduction of its use, together with the large use of exercise and dietary change, may have a positive effect for future body-weight management strategies.

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Appendices

Appendix Questionnaire

Acknowledgments

Supported by grants from the Danish Medical Research Council and the Danish Food Technology and Nutrition Program (FØTEK). We thank the Institute of Market and Opinion Analyses (Sonar) and the daily newspaper Jyllandsposten for kindly giving us access to the data used in this study.